Group messaging issues with Verizon and AT&T

I have seen some other posts regarding this issues but no answers or solutions. When I receive a group message from somebody with AT&T (I phone) and try to respond to the message only people with Verizon can see that I replied. I have tested this several times and people with AT&T can't see that I reply. I can see all the messages people are sending back and forth but my messages to AT&T phones don't show up.